1. Field of the Invention
The present invention relates to evaluating human stress. More particularly, the present invention relates to a method and apparatus for evaluating human stress using photoplethysmography (PPG).
2. Description of the Related Art
In the field of medical diagnosis, there have been various attempts to diagnose cardiovascular diseases, detect a degree of development of a disease, or detect a stiffness of a blood vessel using PPG. PPG indicates a signal corresponding to a quantity of light reflected from a selected part of a human body after being irradiated by light having a particular wavelength emitted from a light source of a light emitting device. Technology using PPG has been primarily developed for the purpose of determining a patient's physiological condition of an arterial system but may be used as an auxiliary means for diagnosing particular diseases.
Conventional apparatuses and methods using PPG have been developed to measure a patient's physiological condition, a condition of a fetus, and a depth of an anesthesia. In addition, a variety of methods using various types of physiological signals to which a human body reacts have been proposed to evaluate human pleasantness, human tranquility, or human stress. In these methods, human stress or pleasantness is measured and evaluated based on at least two physiological signals. In other words, in order to evaluate or continuously monitor the condition of a human body, various physiological signals such as ECG, EEG, EMG, PPG, GSR, and SKT are collected and analyzed.
There are various conventional technologies that may provide reliable analysis results by collecting various physiological signals. However, often there are many restrictions on human subjects along with a requirement of a large scale of a system. For example, a measurement itself often affects a human subject by increasing the subject's level of stress. Moreover, if a glove type or finger contact type measuring devices is used in order to obtain PPG from a human subject's fingers, the human subject is restricted from performing manual operations, such as working on a personal computer (PC) and other operations that require use of the patient's hand.